Monoamine oxidase has been shown to exist as two distinct and separable catalytic activities, MAO A and MAO B. These MAO activities will be studied by resolving the two isoenzymes from extracts of rat liver mitochondria using affinity chromatography. The purified MAO A and MAO B will be compared by their size, lipid and amino acid content, by analysis of proteolytic fragments and by immunological criteria. In addition the MAOs will be compared as membrane bound proteins according to their tendency to form complexes with other membrane proteins and according to their orientation in the outer membrane of mitochondria. The distribution of the MAOs in the brain will be estimated. Biochemical estimates of MAO A and MAO B will be made in neuronal, glial and synaptic ending preparations in seven brain regions. More specific distribution among tracts and nuclei will be estimated using immunofluorescence techniques. The site of hepatic synthesis (free or membrane bound polysomes) of the MAOs will be determined using antibodies to the MAOs, attempts will be made to discern the route MAOs take after their synthesis to their incorporation in the outer membrane.